Ball valves can’t operate on AC voltage, they have capacitors which need to be charged to provide a DC voltage to the motor turning the valve. When capacitor is discharged, it takes a lot of current initially to charge it, this is called “surge current” as there is a big spike at first before things settle down.
Unfortunately, the surge current can be a high as a short circuit (whereas the zone wire is directly connected to the commons, causing the maximum current flow). Rachio protects itself from short circuits by cutting off the zone as soon as possible, otherwise the fuse would trip and cause the whole controller to shut down.
Adding the resistor in series with the ball valve limits the surge current to a value that Rachio does not detect as a short circuit.
Depending on the ball valve, you may have a different issue down the line, whereas the current draw of the valve drops to a low enough value that Rachio thinks that the valve has completely failed. In which case you’ll need a second resistor parallel with the valve. This happens because ball valves use energy only while activating (deactivation is usually handled by the mechanical energy stored in the spring). Once active the current / power draw of the ball valve drops to near zero, as if the valve was not there at all.